High strength hot rolled steel plates and sheets excellent in uniform elongation after cold working and process for producing the same

ABSTRACT

The present invention provides hot rolled steel plates and sheets having a tensile strength of 34 to 62 kgf/mm 2  and excellent in a uniform elongation even after cold working the steel plates and sheets to give round and square tubes, shapes, sheet piles, etc., to such an ordinary degree that the productivity is not lowered, and a process for producing the hot rolled steel plates and sheets. The hot rolled steel plates and sheets contain from 0.04 to 0.25% of C, from 0.0050 to 0.0150% of N and from 0.003 to 0.050% of Ti, also contain from 0.0008 to 0.015% of TiN having a particle size exceeding 1 μm and dispersed in the matrix thereof, and have Ceq. (WES) of 0.10 to 0.45%. The process comprises heating a steel slab containing the constituents as mentioned above to 1,000 to 1,300° C., starting to roll the steel slab subsequently and finishing rolling at a temperature of at least the Ar 3  transformation point, and air cooling from a temperature of at least 500° C. or coiling at a temperature of at least 500° C. and air cooling, the resultant steel structure having a pearlite phase in an amount of 5 to 20% in terms of area fraction.

FIELD OF THE INVENTION

The present invention relates to hot rolled steel plates and sheets forgeneral structure purposes and welded structure purposes excellent inuniform elongation after cold working and having high tensile strength,and a process for producing the same.

BACKGROUND OF THE INVENTION

With the significant progress of the quality and production technologyof hot rolled steel plates and sheets for structure uses in recentyears, demand for steel products excellent in plastic deformability hasincreased more and more, particularly in the field of architecture andcivil engineering from the standpoint of anti-earthquake design, andsteel plates and sheets are now required to have a high strength, a lowyield ratio and a high uniform elongation.

To respond to the requirement, Kokai (Japanese Unexamined PatentPublication) No. 57-16118, for example, discloses a process forproducing electric welded tubes of low yield ratio, for oil wells, inwhich the carbon content is increased to 0.26 to 0.48%, and Kokai(Japanese Unexamined Patent Publication) No. 57-16119 discloses aprocess for producing high tensile strength electric welded tubes of lowyield ratio in which the carbon content is from 0.10 to 0.20%. In eitherof these processes, electric welded tubes requiring no heat treatmentare prepared by producing a hot rolled steel plate or sheet of low yieldratio, and cold working the steel product while the strain amount isbeing restricted so that the amount of work hardening does not becomelarge. Moreover, Kokai (Japanese Unexamined Patent Publication) No.4-176818 proposes a process for producing steel tubes or square tubesexcellent in anti-earthquake properties by hot working a strainlessferrite-pearlite dual phase structure, controlling the cooling rateafter hot working, and heat treating. However, all those processesmentioned above greatly lower the productivity and, in addition, theformer processes markedly impair the weldability. Accordingly, thoseprocesses currently do not necessarily answer the requirements of theindustrial field.

In addition to the disclosures mentioned above, Kokai (JapaneseUnexamined Patent Publication) No. 4-48048 and Kokai (JapaneseUnexamined Patent Publication) No. 4-99248 disclose techniques forimproving the toughness of weld heat-affected region by dispersing oxideinclusions in a steel matrix. The oxide inclusions in the former patentpublication are 0.5 μm or less in particle size and have (Ti, Nb) (O, N)composite crystal phases. The oxide inclusions in the latter patentpublication are 1 μm or less in particle size and have Ti(O, N)composite crystal phases. The techniques of these patent publicationsare essentially different from that of the present invention with regardto dispersion phases and objects.

In general, a steel having a higher strength exhibits a higher yieldratio and a lower ductility, and therefore its uniform elongation islowered. Especially when the steel is cold worked to give round andsquare tubes, shape steels, sheet piles, etc., its uniform elongation ismarkedly lowered because of the influence of work hardening caused bywork strain.

The present invention has been achieved to solve the problems asdescribed above, and an object of the present invention is to providehot rolled steel plates and sheets excellent in uniform elongation andhaving a high tensile strength (at least 34 kgf/mm²) even aftersubjecting them to cold working to give round and square tubes, shapes,sheet piles, etc., to such an ordinary degree that the productivity isnot lowered.

DISCLOSURE OF THE INVENTION

To achieve the objects as described above, the present inventors haveinvestigated in detail the relationship between the chemicalconstituents and crystal structures of steel and mechanical propertiesthereof, the relationship between the mechanical properties of the steelafter cold working and those of the as rolled steel, and the like. As aresult, the present inventors have obtained the following knowledge: inthe case of a steel for general structure uses and welded structure,especially a hot rolled steel plate or sheet having a tensile strengthof 34 to 62 kgf/mm² which is used in the greatest amount in architectureand civil engineering, the relationship between the tensile strength anduniform elongation of as hot rolled product (uniform elongation loweringwith an increase in the tensile strength) approximately agrees with therelationship between them after cold working, and both relationships canbe approximated by the same curve; although both as hot rolled steel andcold worked steel exhibit an increase in the strength and a decrease inthe uniform elongation with an increase in N in the steel, the uniformelongation can be recovered, and a high uniform elongation can beobtained even when the steel has a high strength by further adding Ti,the relationships as mentioned above not holding in this case.

Such knowledge is further illustrated below by making reference to FIG.2.

FIG. 2 is a graph showing relationships between TS (tensile strength,kgf/mm²) and Elu (uniform elongation, %) obtained from as hot rolledsteel products and cold worked ones (square tubes) using steels S-1(comparative example), S-2 (comparative example), T-1 (example) and T-2(example) as listed in Table 1, S-1, T-1 and T-2 being produced byproduction process B as shown in Table 2, and S-2 being produced byproduction process C. The amounts of both Ti and N in S-1 are less thanthe lower limits of the present invention. Though the amount of N in S-2is within the range of the present invention, the amount of Ti is lowand less than the lower limit thereof. In production process C, therolling finishing temperature is low and less than the Ar₃transformation point.

In FIG. 2, with regard to the relationship between TS and Elu, the ashot rolled plates or sheets of S-1 exhibit high TS and Elu. However, thesquare tubes of S-1 exhibit drastically lowered Elu with an increase inTS.

In the case of S-2, the relationship is more significant. The as hotrolled steel plates or sheets exhibit 10% or less Elu when TS is highthough they may exhibit high Elu when TS is low. Square tubes preparedby cold working exhibit 10% or less Elu in most cases, and furtherlowered Elu as TS increases.

That is, in the case of S-1 and S-2, the cold worked steel productsexhibit a tendency of drastically lowering Elu with an increase in TS.

On the other hand, in the case of T-1 and T-2, the as hot rolled steelplates or sheets exhibit almost no lowering of Elu even when TSincreases. Cold worked products obtained therefrom exhibit lowering ofElu to a slight degree, and suffer almost no influence of an increase inTS.

That is, the steel of the present invention containing added N and Ti insuitable amounts exhibits almost no lowering of the uniform elongationwith an increase in the tensile strength even after cold working.Especially, a steel of the invention having TS of at least 47 kgf/mm²can exhibit the effect of the invention. As described above, the steelof the invention has excellent properties as a steel for generalstructural purposes and welded structure.

The present invention has been achieved based on the knowledge asdescribed above, and the subject matter of the invention is highstrength hot rolled steel plates and sheets having a tensile strength of34 to 62 kgf/mm² and excellent in uniform elongation after cold forming,the steel plates and sheets containing from 0.040 to 0.25% of C, from0.0050 to 0.0150% of N and from 0.003 to 0.050% of Ti, also containing0.0008 to 0.015% of TiN having an average size exceeding 1 μm anddispersed in the matrix thereof, and having a Ceq. (WES) of 0.10 to0.45%, the steel plates and sheets being prepared by heating a steelslab containing the constituents as described above to 1,000° to 1,300°C. for hot rolling, rolling the slab, finishing rolling at a temperatureof at least the Ar₃ transformation point, and air cooling the rolledproduct to a temperature of at least 500° C. or coiling the rolledproduct to a temperature of at least 500° C. and air cooling the coiledproduct to form a pearlite phase in the steel structure in an amount of5 to 20% in terms of area fraction, and a process for producing thesame.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A) shows a photomicrograph (magnification: 400) illustrating themetal structure of a flat portion of a square tube obtained from a steelof the invention No. T-2 (MID portion) steel in Table 4 containing 15.2%of a pearlite phase!.

FIG. 1(B) shows a photomicrograph (magnification: 400) illustrating themetal structure of a flat portion of a square tube obtained from acomparative steel No. S-2 steel (thickness (t)=3.2 mm) in Table 4containing 4% of a pearlite phase!.

FIG. 2 shows the relationship between the tensile strength and theuniform elongation of various hot rolled steel sheets and square tubesin Table 4.

BEST MODE FOR PRACTICING THE INVENTION

The present invention is illustrated below in detail.

In the present invention, a low alloy steel slab composed of 0.040 to0.25% of C, 0.0050 to 0.0150% of N, 0.003 to 0.050% of Ti, with a carbonequivalent (Ceq.) being in the range from 0.10 to 0.45%, and the balanceFe and unavoidable impurities is firstly manufactured by a conventionalproduction step of continuously casting molten steel prepared by amelting furnace such as a converter or an electric furnace or making themolten steel to an ingot and blooming the ingot.

In the present invention, constituents in the steel are specified asdescribed above for reasons as described below.

C is an important constituent for determining the strength of steel andthe amount of a pearlite phase in the steel structure. When a hot rolledsteel sheet having a tensile strength of at least 34 kgf/mm² containsless than 5% of a pearlite phase in terms of area fraction in the steelstructure, the uniform elongation after cold forming is markedlylowered. This is because the pearlite engages in the strength of thesteel, prevents an increase in the dislocation density and maintains theplastic deformability. To obtain such a steel structure, the steel isrequired to contain at least 0.04% of C. However, since the weldabilityof the steel is impaired when its C content exceeds 0.25%, the upperlimit of the C content is defined to be 0.25%.

N added to the steel is dissolved in the ferrite matrix, increases thestrength of the steel, and lowers the plastic deformability. However,when N is added together with Ti, TiN is formed. The formation thereofnot only decreases dissolved N in the steel and improves the plasticdeformability but also functions to dispersion strengthen the steel. Nis therefore an important element for imparting high strength and alarge uniform elongation to the steel. To impart such propertiesthereto, it is necessary that TiN having an average particle sizeexceeding 1 μm should be dispersed in the matrix in an amount of 0.0008to 0.015% by weight. To obtain the above TiN, an amount of Ti in therange from 0.03 to 0.050% is effective. Namely, when the averageparticle size of TiN is 1 μm or less, dispersion strengthening is notsufficiently effected.

Moreover, though the necessary amount of N is at least 0.0050%,preferably at least 0.0080%, the strengthening becomes excessive and theuniform elongation is lowered when the amount of N exceeds 0.0150%.Accordingly, the upper limit of the amount of N is defined to be0.0150%. In addition, to effectively form TiN mentioned above in thesteel, it is preferable that the steel should be deoxidized with Aladded thereto prior to the addition of Ti.

Ti is added to the steel of the present invention for reasons asdescribed above, and the amount is preferably from 0.01 to 0.03%.

The carbon equivalent (Ceq.) is obtained by the following formula (basedon WES):

    Ceq.=C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14.

The amount of Ceq. is specified in relation to the strength and theweldability. When the amount is less than 0.10%, sufficient strengthcannot be ensured. When the amount exceeds 0.45%, the weldability isimpaired though a high strength can be obtained. Accordingly, Ceq. isrestricted to the range from 0.10 to 0.45%.

The steel may contain as an effective constituent for improving thestrength and toughness at least one element selected from the groupconsisting of 0.01 to 0.7% of Si, 0.1 to 2.0% of Mn, 0.05 to 1.0% of Ni,0.05 to 1.0% of Cr, 0.02 to 0.5% of Mo and 0.005 to 0.2% of V.

In addition, P and S contained in the steel slab of the presentinvention are harmful impurities which lower the toughness, theweldability, etc. Accordingly, the amount of P and that of S are eachrestricted to 0.025% or less, and P+S is restricted to 0.04% or less.

Furthermore, the steel of the present invention may contain as aneffective constituent for improving the strength and toughness at leastone member selected from the group consisting of 0.05 to 1.0% of Cu,0.005 to 0.05% of Nb, 0.001 to 0.1% of Al, 0.0005 to 0.0020% of B,0.0005 to 0.0070% of Ca and 0.001 to 0.050% of REM (rare-earth metals inlanthanide series including Y).

A steel slab of low alloy steel whose constituents are adjusted in therange as described above is heated to 1,000° to 1,300° C. for thepurpose of hot rolling and is rolled, and the rolling is finished at asteel temperature of at least the Ar₃ transformation point. Theresultant steel is air cooled to a temperature of at least 500° C. toobtain a steel plate, or coiled at a coiling temperature of at least500° C. and air cooled to obtain a hot rolled steel strip.

The lower limit of the heating temperature for hot rolling is defined tobe 1,000° C. to prevent an increase in the strength and a decrease inthe plastic deformability caused as described below: the rollingfinishing temperature of the steel may be less than the Ar₃transformation point depending on the steel thickness; as a result theferrite therein may forcibly be worked, and the dislocation density inthe matrix is then increased. When the steel slab temperature exceeds1,300° C., the yield of the product is markedly lowered due to oxidationthereof. Accordingly, the upper limit is defined to be 1300° C. Therolling finishing temperature is defined to be at least the Ar₃transformation point for reasons as described above. Moreover, for thepurpose of avoiding an unnecessary increase of the strength of the steelplates and sheets of the invention, the air cooling-starting temperatureand coiling temperature after rolling are defined to be at least 500° C.

In the steel plate and sheet produced according to the presentinvention, TiN having an average particle size exceeding 1 μm is finelydispersed and precipitated in the matrix in a proportion of 0.0008 to0.015%. As a result, the steel exhibits a fine grain ferrite-pearlitestructure (partly containing a bainitic structure) containing a pearlitephase in an amount of 5 to 20% in terms of area fraction as shown inFIG. 1(A). Since the steel plate and sheet of the invention have such asteel structure, they are excellent in a uniform elongation after coldworking and have a high tensile strength of 34 to 62 kgf/mm².

EXAMPLES

Examples of the present invention will be illustrated below.

TiN-containing steel slabs having chemical compositions as shown inTable 1 and comparative steels were hot rolled into plates and sheetshaving a thickness of 3.0 to 22.2 mm, and the mechanical properties ofthe resultant steel plates and sheets were investigated. The productionprocesses are shown in Table 2. The properties of the steel productswhich were rolled or worked to have 10% of a strain are shown in Table3.

Tables 4 and 5 show the results of investigating properties of each ofsites in the as rolled steels and square tubes prepared therefrom. FIG.1(A) shows the photomicrograph (x400) of the metal structure of the flatportion (MID) of a square tube prepared from steel T-2 in the presentinvention, and FIG. 1(B) shows that of the metal structure ofcomparative steel S-2. In the steel of the invention in FIG. 1(A), theamount of the pearlite phase was approximately 15.2% in terms of areafraction, whereas the comparative steel in FIG. 1(B) contained it in anextremely small amount of about 4%. FIG. 2 shows the relationshipbetween the tensile strength and the uniform elongation of the steels inthe present invention and that of the comparative steels for comparison,with the results in Table 4 principally utilized.

It is evident from these results that the steels of the presentinvention (C-4, C-6, T-1, T-2, T-3, T-4) maintained a large uniformelongation particularly after cold working though the strength is highcompared with the respective comparative steels. The results can be wellunderstood from FIG. 2 in which there are shown the relationship betweenthe uniform elongation and the strength of the hot rolled plates andsheets of the steels of the invention and the comparative steels and therelationship therebetween of square tubes obtained by cold forming theplates and sheets mentioned above in an actual production line.

                                      TABLE 1    __________________________________________________________________________    (wt %)    Steel         C  Si Mn P  S  Cu Ni Cr Mo V  Al Ti N   Ceq    __________________________________________________________________________    CS      C-1         0.16            0.05               0.45                  0.009                     0.007                        -- -- 0.11                                 0.02                                    -- 0.025                                          -- 0.0027                                                 0.26    CS      C-2         0.16            0.05               0.45                  0.009                     0.017                        -- -- 0.11                                 0.03                                    -- 0.030                                          -- 0.0034                                                 0.26    CS      C-3         0.15            0.05               0.44                  0.010                     0.016                        -- -- 0.07                                 0.02                                    -- 0.026                                          -- 0.0071                                                 0.24    IS      C-4         0.15            0.05               0.45                  0.010                     0.017                        -- -- 0.07                                 0.02                                    -- 0.027                                          0.015                                             0.0071                                                 0.24    CS      C-5         0.08            0.07               0.31                  0.012                     0.017                        0.20                           0.59                              0.06                                 0.10                                    0.01                                       0.027                                          0.001                                             0.0058                                                 0.19    IS      C-6         0.08            0.08               0.28                  0.010                     0.016                        0.21                           0.60                              0.05                                 0.11                                    0.01                                       0.012                                          0.012                                             0.0092                                                 0.18    CS      C-7         0.08            0.07               0.30                  0.010                     0.017                        0.20                           0.57                              0.05                                 0.09                                    0.01                                       0.023                                          0.011                                             0.0167                                                 0.18    CS      S-1         0.14            0.01               0.46                  0.013                     0.003                        -- -- -- -- -- 0.032                                          -- 0.0015                                                 0.22    CS      S-2         0.12            0.09               0.29                  0.016                     0.022                        -- -- 0.05                                 -- 0.005                                       0.038                                          0.001                                             0.0074                                                 0.18    CS      S-3         0.15            0.39               1.40                  0.012                     0.013                        -- -- 0.05                                 -- -- 0.033                                          -- 0.0040                                                 0.41    CS      S-4         0.06            0.04               0.33                  0.009                     0.010                        -- -- 0.03                                 -- -- 0.034                                          -- 0.0110                                                 0.12    IS      T-1         0.15            0.09               0.27                  0.014                     0.019                        -- -- 0.05                                 -- 0.006                                       0.039                                          0.016                                             0.0111                                                 0.21    IS      T-2         0.17            0.09               0.28                  0.011                     0.015                        -- -- 0.06                                 -- 0.007                                       0.030                                          0.021                                             0.0110                                                 0.23    IS      T-3         0.15            0.38               1.39                  0.013                     0.013                        -- -- 0.06                                 -- -- 0.031                                          0.022                                             0.0100                                                 0.41    IS      T-4         0.05            0.05               0.39                  0.010                     0.010                        -- -- 0.06                                 -- -- 0.031                                          0.027                                             0.0090                                                 0.13    __________________________________________________________________________     Note:     CS = Comparative steel; IS = Steel of the present invention     Ceq. (WES) = C + Si/24 + Mn/6 + Ni/40 + Cr/5 + Mo/4 + V/14

                  TABLE 2    ______________________________________    (Temperature: °C.)            Steel slab                     Rolling    Production            heating  finishing                              Steel sheet                                       Air cooling    process temp.    temp.    coiling temp.                                       starting temp.    ______________________________________    IS A    1200     950      680      --    IS B    1230     880      630      --    CS C    1230      790*    490      --    IS D    1180     900      --       700    ______________________________________     Note:     (1) IS = Steel of the present invention; CS = Comparative steel     (2) *Temperature being lower than the Ar3 transformation point

                                      TABLE 3    __________________________________________________________________________             Thick-             ness                 YS1*  TS*   El*                                ELu*    Steel        Process             (mm)                 (kgf/mm.sup.2)                       (kgf/mm.sup.2)                             (%)                                (%) Note    __________________________________________________________________________    CS C-1        A    5.7 31.1  43.0  42.0                                22.2                                    as rolled             5.4 48.5  48.5  28.0                                7.2 10% strained    CS C-2        A    5.7 29.2  43.7  43.5                                21.6                                    as rolled             5.4 49.3  49.8  26.0                                5.2 10% strained    CS C-3        A    5.7 31.2  44.8  40.5                                21.0                                    as rolled             5.4 52.1  52.8  23.0                                2.0 10% strained    IS C-4        A    5.7 32.6  46.0  44.0                                20.0                                    as rolled             5.4 52.6  53.3  31.0                                9.0 10% strained    CS C-5        A    8.5 24.5  34.6  47.0                                22.8                                    as rolled             6.9 42.4  43.3  21.0                                1.2 10% strained    IS C-6        A    8.7 25.0  35.4  45.5                                22.0                                    as rolled             6.9 43.5  46.3  26.0                                6.4 10% strained    CS C-7        C    8.5 41.5  48.8  34.0                                17.5                                    as rolled             6.9 57.0  57.8  20.1                                1.4 10% strained    __________________________________________________________________________     Note:     IS = Steel of the present invention; CS = Comparative steel     pieces being prepared in accordance with the JIS Z 2201 5 test piece

                                      TABLE 4    __________________________________________________________________________             Thick-             ness                 YS1*  TS*   El*                                ELu*    Steel        Process             (mm)                 (kgf/mm.sup.2)                       (kgf/mm.sup.2)                             (%)                                (%) Note    __________________________________________________________________________    CS S-1        B    3.2 31.3  45.3  39.0                                19.2                                    as rolled             3.3 45.0  47.8  33.2                                11.6                                    sq. tube F        B    3.2 31.8  45.9  39.2                                18.8                                    as rolled             3.2 38.4  46.3  36.0                                17.3                                    sq. tube F        B    6.0 31.9  44.7  40.6                                19.7                                    as rolled             6.1 40.4  45.3  37.0                                14.5                                    sq. tube F    CS S-2        C    3.2 34.0  44.6  34.8                                16.3                                    as rolled             3.2 48.1  51.5  20.4                                4.0 sq. tube F        C    6.0 39.8  48.1  29.0                                9.8 as rolled             6.0 46.3  50.8  23.6                                4.9 sq. tube F        C    5.7 31.7  44.2  38.0                                18.7                                    as rolled             5.8 43.2  48.7  29.0                                5.5 sq. tube F    IS T-1        B    3.0 32.1  45.3  39.5                                19.5                                    as rolled (TOP)             3.1 38.7  46.8  36.0                                16.6                                    sq. tube F (TOP)        B    3.0 30.3  46.4  40.0                                20.0                                    as rolled (MID)             3.1 38.1  47.1  36.5                                17.0                                    sq. tube F (MID)        B    3.1 34.4  51.2  34.0                                17.5                                    as rolled (BOT)             3.1 42.8  51.8  31.0                                13.6                                    sq. tube F (BOT)        B    3.1 65.3  71.9  28.0                                6.2 sq. tube C (BOT)             3.1 60.9  65.5  32.0                                7.9 sq. tube C (BOT)    IS T-2        B    3.0 34.7  48.9  40.0                                19.8                                    as rolled (TOP)             3.1 38.4  48.2  35.0                                16.0                                    sq. tube F (TOP)        B    3.0 30.9  47.3  37.0                                19.4                                    as rolled (MID)             3.1 38.8  48.1  35.0                                16.2                                    sq. tube F (MID)        B    3.1 33.3  52.9  35.0                                17.6                                    as rolled (BOT)             3.1 39.4  49.0  35.0                                16.0                                    sq. tube F (BOT)        B    3.1 60.8  67.4  33.0                                12.0                                    sq. tube C (BOT)             3.1 59.3  66.5  35.0                                12.3                                    sq. tube C (BOT)    __________________________________________________________________________     Note:     IS = Steel of the present invention; CS = Comparative steel     tensile test pieces being prepared in accordance with the JIS Z 2201 5     test piece except for corner portions test pieces of which were prepared     in accordance with the JIS Z 2201 12B test piece     *tubes of each of the types having each a dimension of 75 mm × 75 m

                                      TABLE 5    __________________________________________________________________________             Thick-             ness                 0.2% PS*                       TS*   El*                                ELu*    Steel        Process             (mm)                 (kgf/mm.sup.2)                       (kgf/mm.sup.2)                             (%)                                (%) Note    __________________________________________________________________________    CS S-3        D    22.2                 36.0  54.9  28.4                                20.0                                    as rolled             22.0                 38.1  56.0  24.7                                16.6                                    sq. tube F             22.1                 57.1  66.2  15.0                                5.2 sq. tube C    CS S-4        C    9.0 30.0  43.0  40.0                                17.5                                    as rolled             9.1 38.2  45.8  35.0                                9.5 sq. tube F             9.0 48.9  54.1  26.0                                4.2 sq. tube C    IS T-3        D    22.1                 36.2  55.1  29.0                                21.3                                    as rolled             22.0                 38.5  56.2  27.1                                18.7                                    sq. tube F             22.0                 57.3  66.3  20.6                                12.7                                    sq. tube C    IS T-4        B    8.9 29.3  45.0  38.5                                20.5                                    as rolled             9.0 34.2  45.3  38.0                                19.6                                    sq. tube F             9.0 50.3  56.5  36.0                                16.0                                    sq. tube C    __________________________________________________________________________     Note:     IS = Steel of the present invention; CS = Comparative steel     S3, T3: square tubes each having a dimension of 350 mm × 350 mm,     tensile test pieces being prepared in accordance with the JIS Z 2201 1B     test piece     S4, T4: square tubes each having a dimension of 250 mm × 250 mm,     tensile test pieces being prepared in accordance with the JIS Z 2201 5     test piece

POSSIBILITY OF UTILIZATION IN THE INDUSTRY

As described above, in the present invention, high strength hot rolledsteel plates and sheets having a tensile strength of 34 to 62 kgf/mm²and extremely excellent in a uniform elongation even after beingsubjected to cold forming to such a degree that the ordinaryproductivity is not lowered can be produced by specifying theconstituents in the steel to form relatively large TiN particles havinga dispersion strengthening capability, and forming an effective pearlitephase therein. The high strength hot rolled plates and sheets areextremely useful as steel products for general structure uses and weldedstructure, and materials for round and square tubes, shapes, sheetpiles, etc.

We claim:
 1. High strength hot rolled steel plates and sheets excellentin uniform elongation after cold working, containing from 0.04 to 0.25%by weight of C, from 0.0050 to 0.0150% by weight of N and from 0.003 to0.050% of Ti, having a carbon equivalent (Ceq.) defined by the formuladescribed below of 0.10 to 0.45% and a pearlite phase in an amount of 5to 20% in terms of area fraction, and containing from 0.0008 to 0.015%by weight of TiN having an average particle size exceeding 1 μm anddispersed therein:

    Ceq.=C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14.


2. The high strength hot rolled steel plates and sheets according toclaim 1, wherein the tensile strength is from 34 to 62 kgf/mm².
 3. Thehigh strength hot rolled steel plates and sheets according to claim 1,wherein the plates and sheets contain at least one constituent selectedfrom the group consisting of 0.01 to 0.7% by weight of Si, 0.1 to 2.0%by weight of Mn, 0.05 to 1.0% by weight of Ni, 0.05 to 1.0% by weight ofCr, 0.02 to 0.5% by weight of Mo and 0.005 to 0.2% by weight of V. 4.The high strength hot rolled steel plates and sheets according to claim1, wherein the plates and sheets further contain at least oneconstituents selected from the group consisting of 0.05 to 1.0% byweight of Cu, 0.005 to 0.05% by weight of Nb, 0.001 to 0.1% by weight ofAl, 0.0005 to 0.0020% by weight of B, 0.0005 to 0.0070% by weight of Caand 0.001 to 0.050% by weight of REM.
 5. The high strength hot rolledsteel plates and sheets according to claim 1, wherein the contents of Pand S are controlled to satisfy the following conditions:P≦0.025% byweight, S≦0.025% by weight, and P+S≦0.04% by weight.
 6. A process forproducing high strength hot rolled steel plates and sheets excellent inuniform elongation after cold working, which comprises heating a steelslab containing from 0.04 to 0.25% by weight of C, from 0.0050 to0.0150% by weight of N and from 0.003 to 0.050% by weight of Ti, andhaving a carbon equivalent (Ceq.) of 0.10 to 0.45% to a temperature from1000° to 1300° C., starting to roll the heated steel slab and finishingrolling at a temperature of at least the Ar₃ transformation point, andair cooling from a temperature of at least 500° C.
 7. The processaccording to claim 6, wherein a plate is produced by air cooling theproduct from a temperature of at least 500° C.
 8. The process accordingto claim 6, wherein a steel strip is produced by coiling the product ata temperature of at least 500° C. and air cooling.